DESCRIPTION Reversible protein phosphorylation is involved in a diverse array of biological processes, including metabolism, cell proliferation and differentiation, membrane transport, gene expression, locomotion, memory, and learning. Yet very little is known about the biophysical mechanisms at work in regulation of protein function by phosphorylation. The goal of the proposed research is to elucidate these mechanisms, using the Src Homology 2 (SH2) domain from chicken c-Src as a model system. The main objectives are i) to determine the structural, dynamic, and functional effects of phosphorylation of a specific Tyrosine residue (Y213) within the SH2 domain that has been observed in nature to alter ligand specificity, and ii) to ascertain if this specificity switch can be modified in a cell cycle dependent or tissue specific manner by incorporating a Ser or Thr trigger mechanism. Multi-dimensional NMR spectroscopy will be employed for structure determination and dynamic characterization of this model system. Titration microcalorimetry will be used as a functional assay to evaluate changes in ligand specificity and affinity. The combination of these experiments will help provide a comprehensive understanding of the biophysical effects of reversible protein phosphorylation on Src SH2 specificity switching. This information can be used to design proteins that, when coupled with gene therapy, will allow the introduction of molecular on/off switches in specific stages of the cell cycle or in different tissue types. This approach has great potential in aiding development of novel anti-cancer therapeutics.